Constructing preforms from capillaries and canes

ABSTRACT

The invention relates to a method of producing a preform for a holey optical fibre, and more particularly, to a method of producing polymer holey optical fibre using novel capillary and cane designs that allow a construction of complex holey structures. The capillaries may have a complex internal structure including multiple holes, holes of non-circular shape, off-centre holes, holes of different sizes, or any combination of these. The canes may have a complex external shape to define interstitial holes when the canes arc combined in a stack (see FIG.  1 ). The capillaries and canes may be made of different materials and combined within the same structure.

FIELD OF THE INVENTION

This invention relates to a method of producing a preform for a holeyoptical fibre, and more particularly, to a method of producing capillaryand cane designs that can be combined into a stack and fused to allowthe construction of complex structures that will be of use forwave-guiding applications.

The invention also relates to novel capillary and cane designs for waveguiding applications, and in particular to capillary and cane designsthat can be combined and joined so as to allow the construction ofcomplex structures for use in wave guiding applications, and inparticular for the production of photonic crystal optical fibres.

BACKGROUND TO THE INVENTION

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

There is considerable interest in making complex holey structures thatcan subsequently be drawn into a microstructured fibre. This iscurrently done by stacking and fusing capillaries or canes.

Photonic crystal fibres are commonly made in glass. The fabricationtechnique used is capillary stacking with thermal fusing. The generaltechnique used has been to use glass canes or capillaries having asingle central hole. Canes are solid rods, without any internal holes,whilst capillaries are elongated structures that have one or more holesrunning through them. The capillaries used to date have usually been ofcircular cross-section, and had a single central hole. There have been anumber of examples in which holes of different sizes were combined inone structure, but this has generally proved difficult because theself-packing properties of same-sized rods and canes breaks down whendifferent diameters are used. In this case there is a need to “pack” thestructure to support it, and prevent it collapsing. Maintaining the holestructure in glass holey fibres is very difficult, and requires a highdegree of control over both the preform fabrication and drawingprocesses. It is likely that for this reason very few structures thatdeviate from the same-sized canes or single holed capillary stack havebeen reported. In addition, any deviation from the single central holedcapillary requires control of the rotational orientation of thecapillary.

Whilst attempts have been made to use glass canes and capillaries ofhexagonal cross-section with multiple holes, difficulties with fusingand distortion have been encountered resulting in poor quality fibre.

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

SUMMARY OF THE INVENTION

To this end, one aspect of the present invention provides a method ofproducing a preform for a holey optical fibre comprising providing aseries of preform elements adapted to be connected together to constructsaid preform with a series of holes therein, each hole being formed inan element or formed by combining two or more elements.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”.

A further aspect of the present invention provides a preform for a holeyoptical fibre, said preform comprising providing a series of preformelements to be connected together to construct said preform with aseries of holes therein, each air hole being formed in an element orformed by combining two or more elements.

A further aspect of the present invention provides a method of producinga preform for a polymer holey optical fibre comprising providing aseries of preform elements adapted to be connected together to constructsaid preform with a series of holes therein, each hole being formed inan element or formed by combining two or more elements.

A further aspect of the present invention provides a preform for apolymer holey optical fibre, said preform comprising providing a seriesof preform elements to be connected together to construct said preformwith a series of holes therein, each air hole being formed in an elementor formed by combining two or more elements.

Preferably, the rotational orientation of the capillaries and caneswithin the stack can be controlled by means of stackable cross-sections,such as hexagonal, triangular or rectangular cross-sections, and/or bymeans of a slotted structure.

In the context of this specification it should be noted that the terms“capillaries” and “canes” are not to be associated with a particularsize. For example, in the context of this specification, capillaries andcanes may have dimensions up to the order of centimetres in diameter.The capillaries may have an internal structure of one or more holes,which may or may not be circular, and which may or may not be of thesame dimensions. The canes may have a complex external cross-section todefine the shape of the interstitial holes.

The canes or capillaries may be made of a variety of materials andcombined within the same structure.

The canes and capillaries may fit together either concentrically, orwithin stacked structure, or a combination of the two. They may then befused together either prior to drawing the fibre, or as the preform isdrawn to fibre.

Standard sized capillaries and canes containing different holestructures, canes, and possibly canes and capillaries made of differentmaterials could be used as “Leg® pieces” to rapidly construct a verylarge range of structures. This approach lends itself to automation ofthe assembly process.

Using the range of capillary and cane designs described here it ispossible obtain any desired cross-section for a preform for an opticalfibre. The cross-section may include a complex pattern of holes, whichmay be periodic, and may additionally include controlled variations inthe material properties.

It is to be noted that the various aspects of the present invention maybe used to produce a variety of photonic structures, including those fornon-optical wavelengths.

Advantageously, considerable complexity can be introduced into a preformstructure and resulting optical fibre by varying the external shape ofthe capillaries and canes to affect the stacking properties, changingthe internal pattern of the holes within the capillaries, for example byusing off-centre holes, holes of different sizes, multiple holes, andholes of non-circular shape, or any combination of these. Complex holeystructures can be obtained either by combining capillaries, or canes, ora mixture of the two. The resulting hole structure will combine theholes in the capillaries with interstitial holes. Canes can be designedto have an external shape such that when they are fitted together theinterstitial holes have a particular geometry, for example, theinterstitial holes may be circular. The advantage of this approach isthat it is usually easier to produce solid structures rather than voidedstructures.

Additionally, the methodology of the present invention enables the useof relatively large cane and capillary elements. This in turn aids inthe handling and positioning of the elements when constructing a preformof the desired structure. Furthermore, by using relatively large caneand capillary elements (for example one centimetre or greater indiameter) some degree of deformation of the individual elements can beaccommodated without deforming the main structure of the preform.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 illustrates a circular interstitial hole structure formed by agroup of adjacent canes;

FIGS. 2 a to 2 d illustrate examples of the types of holes which may beincluded in capillaries as a result of the present invention; and

FIG. 3 illustrates a concentric capillary formation which can be formedby the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Whilst the following description of the various aspects of the inventionare directed to preforms formed from polymer materials, it is to benoted that where applicable the various aspects of the invention canalso be employed in the manufacture of preforms from other materials orcombination of materials, including glass.

One aspect of the present invention is directed to capillary and canedesigns that will allow the fabrication of complex structures for use inwave guide applications. In particular, the canes and capillaries can beused to produce preforms for photonic crystal fibres.

A fundamental problem to be addressed in the use of complex capillariesis the control of rotational orientation. The problem is addressed byusing two approaches: stackable cross-sections and slotted structuresthat hold the canes or capillaries in a particular defined position. Itis to be noted that these two features (stackable cross-sections andslotted structures) may not be required for the whole length of thepreform. The stacking and slotting may be restricted to a part of thepreform that is not drawn but serves to hold the canes and capillariesin place. The structure may also be held in place by external means,which allows for a structure where all or some of the canes orcapillaries do not touch the other elements. Control of the positionalorientation can also be achieved by means of slotted structures thathold the canes or capillaries in a defined position.

In one embodiment, the desired holes running through the preform areformed in separate elements, for example, in a capillary tube. Inanother embodiment, the holes are formed by the combination of one ormore adjacent elements. An example of a circular hole structure formedusing canes is depicted in FIG. 1.

The elements themselves can be in the form of longitudinally extendingelements such as the capillaries and canes mentioned above. In analternative embodiment, laterally extending plates or discs, with orwithout hole structure, can be stacked so as to form the preform.

Each element forming the preform can be produced from an identicalmaterial or from a different material. The elements themselves can be ofa standard size and shape and connected together in a conventionalfashion, such as by means of tongue and groove, or pin and slot, so asto form the desired preform and array of holes therethrough. Such aconfiguration allows for the fixing of the rotational and positionalorientation of the capillary or cane element. Advantageously, such aconstruction lends itself to automated production of each element andthe preform itself.

Alternatively, for specific or complex applications individuallydesigned elements can be prepared which, when combined, form the desiredpreform structure.

The preform elements can be prepared by conventional techniques. Forexample, with polymer material these include, but are not limited to,casting, extrusion, injection moulding, or similar.

A variety of chemical or physical fusing techniques may be used to formthe preform from the individual elements.

For polymers, one such technique is chemical fusing wherein a monomericmaterial or polymeric/monomeric mixture is used to surround the airholes formed by the various elements. Preferably, the air holes aresealed to avoid ingress of the monomeric or polymeric/monomeric mixture.This technique applies to air holes formed in a single element or airholes formed by co-joined elements.

In another process, solvent is applied to soften the exterior surface ofeach element, the elements being fused by pressing them together. Thesolvent can be applied in fluid, gaseous or vapour form.

Another alternative is thermal fusing where heat is applied to thevarious elements. This can be combined with pressing the elementstogether. Alternatively, such pressure can arise from the thermalrelaxation of stress that is present in the elements, which can lead toa controlled amount of expansion of the individual elements. Ifexpansion of the overall structure is restricted, this will result inthe pressing of the elements together. Physical fusing can also beapplied by, for example, application of irradiation to the elements.

It should also be recognised that each of these techniques can beassisted by providing, for example, a sleeve of polymeric material in towhich the elements are placed. As will be appreciated by persons skilledin the art, upon polymerisation, such a polymeric material may shrink.This may in itself serve to provide or assist binding of the variouselements of the preform.

The rotational orientation of the capillaries and canes within the stackcan be controlled by means of “stackable” cross-sections (such ashexagonal, triangular or rectangular), or by means of a slottedstructure.

The capillaries may have an internal structure of one or more holes,which may not be circular, or of the same dimensions. The canes may havea complex external cross-section to define the shape of the interstitialholes.

The canes or capillaries may be made of a variety of materials andcombined within the same structure. Useful material variations inpolymers include different transparent polymers, grafted polymers,polymers with inclusions, doped polymers, porous polymers, and liquidcrystal polymers. Similarly, it is possible to use glass materialsincluding, for example, glasses with different dopants, and glasses withdecreased viscosity at lower temperatures.

The canes and capillaries may be fitted together either concentrically,or within stacked structure, or a combination of the two andsubsequently fused together. It should be noted that the possibility ofcasting material around the capillaries or canes means that in principlethe structures do not have to be stackable.

Standard sized capillaries and canes containing different holestructures, canes, and possibly canes and capillaries made of differentmaterials could be used to rapidly construct a large range ofstructures. This approach lends itself to automation of the assemblyprocess.

Using the range of capillary and cane designs described here it ispossible to obtain almost any desired cross-section for a preform for anoptical fibre. The cross-section may include a complex pattern of holes,which may be periodic, and may additionally include controlledvariations in the material properties.

Polymers offer certain advantages over glass. They are more easilyprocessed, so that a number of techniques can be used to make the canesor capillaries. Polymers can be readily cast or extruded, in addition tobeing drilled and machined. For example, the capillaries or canes couldbe “sleeved”. It should be noted that the external surface area tovolume ratio of the capillaries is important in the fusing process. Forfusing to occur without distortion of the hole structure, the layer thatis softened or modified in the fusing process must not compromise thematerial that defines the hole structure. Preferably, the fusing methodonly affects the capillaries or canes to be fused in a superficiallevel, without affecting the internal structure. The capillaries orcanes formed in these ways can then be drawn down, if necessary, to thedesired dimensions. Once the capillaries or canes are formed, there area large variety of ways in which they can be fused, unlike the singlemethod (thermal fusing) used for glass. The possibility of chemicalfusing, including casting around the canes or capillaries greatlyenhances the number options available in polymers. For example, bycasting around the canes or capillaries, it is possible to combineobjects that do not stack neatly, which is impossible for glass. Finallythe composition of polymers can be varied in many more ways than glass,and by tailoring properties such as molecular weight and additives, itis possible to adjust the rheology of different polymers so that theymaintain compatibility during drawing. Doping, grafting and inclusionscan all be used to modify the optical and other properties of the canesand capillaries. Examples of modifications that may be considered areinclusions so as to modify the refractive index, non-linear opticalpolymers, conducting polymer systems, polymers with gain properties(suitable for use in lasers), and polymer systems with enhanced electroand magneto-optic performance.

Novel aspects of the capillary and cane designs of the present inventionare as follows:

Stackable External Cross-Sections

Whilst attempts have been made to use glass canes and capillaries ofhexagonal cross-section with multiple holes, difficulties have beenencountered. For example, the many holed glass capillary suffers fromshrinking with respect to the hexagonal canes, leading to poor fusing ofthe canes and capillaries. Furthermore, the outer hexagonal surfacesbecome distorted, thereby adversely affecting the packing of the canesand capillaries. As a consequence, the resulting fibre is of poorquality.

Advantageously however, the present invention enables the use ofstackable capillaries with a pattern of holes that have the appropriatesymmetry it is possible to make the stack relatively insensitive torotational orientation. For example by using a hexagonal externalcross-section the hexagons will tend to align themselves and packneatly, without spaces between adjacent capillaries. Similarly, canesthat have a non-circular cross-section can be made stackable by usingsuitable shapes (eg. square, triangular or hexagonal, with modificationsto produce the desired hole structure).

By using other stackable capillary designs, such as a square ortriangular cross-section, it is also possible to vary the symmetry ofthe lattice structure.

A variety of photonic crystal components can also be envisaged, forwhich control of possible twisting and bending in the fibre would beessential. For these applications it would be advantageous draw thecomponents from a preform with a suitable cross-section (for example,rectangular) so that they could be easily packaged.

Slotted Capillaries and Canes

The shape of the capillaries and canes may be designed so that theyeither slot together like a jigsaw or into another structure. Forexample hexagonal capillaries may be slotted into a cast honey-combstructure, and then be fused together. This allows both their positionin the whole structure and rotational orientation to be tightlycontrolled.

It should be noted that the slotted structure does not need to belocated in the section of the preform which is drawn.

Multiple Holed Capillaries and Related Canes

Once the problem of positional and rotational orientation is addressed,it immediately becomes feasible to have many-holed capillaries. Theseare desirable because they offer a way of rapidly assembling the largenumber of holes and complexity of structures needed for many photoniccrystal fibre applications. A small number of many-holed capillaries canbe arranged and fused more readily and accurately than a large number ofsingle holed capillaries. Multiple holed capillaries should have across-section which is inherently stackable, or which is held in placeby a suitable structure such as tongue and groove mechanism or externaljig. Stackable cross-sections include hexagonal, triangular,rectangular, ribbon structures etc. Similarly, canes designed to giveinterstitial holes can be designed to define multiple holes incombination, for example by using a ribbon structure in which a largenumber of half-holes. Similarly, “ribbon” capillaries, similar to ribbonwire, could also be easily stacked.

In terms of chemical fusing for example, the issue of wetting thecapillaries is an issue when trying to cast around the structure. Thelarger the capillary, the simpler it is to cast around it.

Size and Shape Variations in the Hole Structures

An additional level of complexity can be introduced to the structureonce the positional and rotational orientation is fixed, by allowingsize and shape variations in the hole structure. By restricting theposition locking to a region of the preform to the part that is notdrawn, it is possible for capillaries and canes of any cross-section tobe combined in the drawing region. In other words the canes orcapillaries can have any external shape and any pattern of holes,provided that their position is adequately locked in some region of thepreform.

Examples of some of the types of holes which can be included incapillaries include off centre holes, non circular holes, differentsized holes etc. Examples of different types of hole structures areillustrated in FIGS. 2 a to 2 d.

Such structures have many possible uses in photonic crystal fibres.Elongated holes for example could be used to build up approximations toring structures or reduce bending losses. Different sized holes could beused to establish multiply periodic structures, which have for example,more an extended band gap, or several band gaps. Changes in thecross-section can also be used to design cross-sections for particularapplications such as polarisation and dispersion control, couplers etc.

Concentric Cylindrical Capillaries

Concentric capillaries with or without protrusions on their outersurface can be arranged to form structures that are, or closelyapproximate, Bragg structures. This allows for the fabrication of Braggfibres in which the guiding is achieved by Bragg reflection off themulti-layered concentric cladding of the fibre. Fusing could occur whereadjacent cylinders touch each other. By varying the composition of thecylinders, or treating their surfaces, it is possible to change theiroptical properties. Protrusions extending from the capillaries can beused to allow nearly complete air rings to be formed. FIG. 3 illustratesthe way that nearly complete air rings could be formed by this process.

In a further development, capillaries which include hole structures areconcentrically stacked.

Twisted Structures

The large refractive index contrast found in holey fibres makes themparticularly suitable for controlling the polarisation properties of theresulting structures. Spinning or twisting the fibre as is it is beingdrawn means that circularly birefringent or low birefringence fibres canbe produced.

Combinations of Materials

Capillaries and canes of different composition may also be combined inthe stack that forms the preform. In particular the use of dopedmaterials, nano-composite materials for example using rare-earth ormetallic inclusions, birefringent and porous materials may be used.

Polymers allow a huge variety of compositions to be used. Polymers ofdifferent refractive indices may be combined, or polymers that aregrafted or doped with new functionalities, such as dyes, photochromicmaterials, birefringent materials etc. Most of these materials decomposeat the processing temperatures used for glasses. Using liquidcrystalline polymers it is possible to produced molecular ordering ofthe material, which would be useful for example for polarisationcontrol. Polymers for example can be made chiral, a possibility thatdoes not exist for glass. Nano-composite materials may also be used inwhich inclusions such as metals, rare earths, high refractive indexmaterials etc. are used to modify properties such as opticalnon-linearity, magneto-optic effect, the electro-optic effect, the gainof the material, the electrical conductivity, etc. By modifying polymerproperties such as molecular weight etc. it is possible to ensure thedrawing compatibility of different polymeric materials—a feature that isnot available in glass. In other materials, such as glasses, a morelimited range of chemical modifications are possible. However, the useof doped material, for example by way of modified chemical vapourdeposition or chelation, is possible.

Additionally, techniques such as sleeving and coating are applicable soas to produce capillaries and canes formed from combinations ofmaterials.

Combination of Structures and Composition

All of the canes and capillaries described here can be combined to formcomplex structures. For example, capillaries and canes of standardexternal cross-section but differing internal cross-section andcomposition could be combined like “Lego™ pieces” to rapidly constructcomplex structures. This offers the possibility to completely controlthe cross-sectional of a preform, including control of the materialproperties. An attractive feature of this approach is that it lendsitself to automation. Using a set of standard compatible canes andcapillaries it is possible to build up structures “to order”.

Additionally, it is possible to form capillaries and canes fromcombinations of different materials. For example, it is possible toproduce capillaries or canes with multiple layers of different materialsby techniques such as sleeving or coating or other techniques.

It is also to be noted that capillaries and rods and preforms of complexstructures and composition may be constructed by successive drawing andsleeving as well as by stacking, a process which allows the size of thefinal structure to be easily controlled Capillaries and rods formed bythis technique can subsequently be included in a stacked structure.

There are a large number of applications in which the composition andstructure can both be varied to obtain particular optical outputs. Theseapplications define a series of hybrid fibres, which combine photoniccrystal fibre structures with variations in composition that may beproduced by conventional processing techniques, or by simply includingcapillaries or canes of different composition. Some fibres of interestthat could be produced by this approach include:

-   -   single or multiple core photonic fibres in which with doping in        the inner regions to generate particular transverse modes for        laser, amplifier and possibly dispersion control applications.    -   Bragg hole fibres that have chirality added (for example by        twisting) and dopants to produce a circularly birefringent laser        output when feedback is added.    -   use of Bragg structures with appropriate spacings in photonic        fibres to obtain Fresnel waveguides.        Variations along the Length of the Preform

The same fusing and assembly techniques can be applied to producevariations along the length as well as across the cross-section of thepreform. This may be used to produce variations in structure, such aschirality, material, or simply to extend the length of the preform.

The preform may comprise a plurality of interconnected capillaries andcanes. When connected together, these capillaries and canes combine toform a preform suitable for the production of holey fibres. It is to benoted that the capillaries and canes may be predrawn.

The designs described here address the problems of maintainingpositional and rotational orientation of the capillaries and canes,which in turn allows multiple hole structures, non-circular holes andcanes or capillaries of different composition to be combined. Inaddition, the present invention provides a way of simply producing orapproximating ring structures used for Bragg fibres. The combination ofthese techniques allows unprecedented control of the cross-sectionalarrangement and composition of the fibre. The designs described alsoallow a methodology that can be easily automated, and can produce easilycustomisable structures.

Some of the many applications that would use the sorts of structurespossible with the novel capillary and cane design of the presentinvention are:

-   -   low bending loss fibres. It is known that increasing the hole        size as a function of distance from the core will reduce bending        losses    -   extended bandgap or multiple band gap fibres    -   fibres for dispersion control    -   fibres for polarization control    -   fibres with Bragg structures    -   multi-core fibres and multi-structure fibres, in which more than        one light guiding area is created within the fibre    -   fibres for high optical non-linearity, electro- and        magneto-optic effect, fibres with electrically conducting        elements, gain etc.    -   fibre sensors.

Although the invention has been described with reference to specificexamples it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

1. A method of producing a preform for a holey optical fibre comprisingproviding a series of preform elements to be connected together toconstruct said preform with a series of holes therein, each hole beingformed in an element or formed by combining two or more elements.
 2. Amethod of producing a preform for a holey optical fibre comprisingproviding a series of preform elements adapted to be connected togetherto construct said preform with a series of holes therein, each holebeing formed in an element or formed by combining two or more elements.3. A method of producing a preform for a polymer holey optical fibrecomprising providing a series of preform elements adapted to beconnected together to construct said preform with a series of holestherein, each hole being formed in an element or formed by combining twoor more elements.
 4. A method of producing a preform for a polymer holeyoptical fibre comprising providing a series of preform elements to beconnected together to construct said preform with a series of holestherein, each hole being formed in an element or formed by combining twoor more elements.
 5. The method as claimed in any one of claims 1 to 4wherein the holes running through the preform are formed in separateelements.
 6. The method as claimed in any one of claims 1 to 4 whereinthe holes are formed by the combination of one or more adjacentelements.
 7. The method as claimed in any one of claims 1 to 6 whereineach element forming the preform is produced from an identical material.8. The method as claimed in any one of claims 1 to 6 wherein saidelements forming the preform are produced from different materials. 9.The method as claimed in any one of the preceding claims wherein theelements fit together either concentrically, or within a stackedstructure, or a combination of the two.
 10. The method as claimed in anyone of the preceding claims wherein said elements are fused togetherprior to drawing the fibre.
 11. The method as claimed in claim 10wherein said elements are thermally fused together prior to drawing thefibre.
 12. The method as claimed in claim 10 wherein said elements arechemically fused together prior to drawing the fibre.
 13. The method asclaimed in any one of the preceding claims, wherein the rotationalorientation of the capillaries and canes within the stack is controlledby means of stackable cross-sections.
 14. A preform for a holey opticalfibre, said preform comprising a plurality of preform elements connectedtogether to construct said preform with a series of holes therein, eachair hole being formed in an element or formed by combining two or moreelements.
 15. A preform for a polymer holey optical fibre, said preformcomprising a plurality of preform elements connected together toconstruct said preform with a series of holes therein, each air holebeing formed in an element or formed by combining two or more elements.16. The preform as claimed in claim 14 or 15 wherein the holes runningthrough the preform are formed in separate elements.
 17. The preform asclaimed in claim 14 or 15 wherein the holes are formed by thecombination of one or more adjacent elements.
 18. The preform as claimedin claim 14 or 15 wherein each element forming the preform is producedfrom an identical polymeric material.
 19. The preform as claimed inclaim 14 or 15 wherein said elements forming the preform are producedfrom different materials.
 20. The preform as claimed in claim 14 or 15wherein said elements forming the preform are produced from differentmaterials.
 21. The preform as claimed in claim 14 or 15 wherein theelements are fitted together with concentrically, or within a stackedstructure, or a combination of the two.
 22. An optical fibre formed froma preform as claimed in any one of claims 14 to
 21. 23. A single coreoptical fibre formed from a preform as claimed in any one of claims 14to
 20. 24. A multiple core optical fibre formed from a preform asclaimed in any one of claims 14 to
 20. 25. A Bragg fibre formed from apreform as claimed in claim 14 or
 15. 26. A Fresnel wave guide formedfrom a preform as claimed in claim 14 or
 15. 27. A birefingent opticalfibre formed from a preform as claimed in claim 14 or
 15. 28. A fibrecoupler formed from a preform as claimed in claim 14 or
 15. 29. Elementsof a fibre sensor formed from a preform as claimed in claim 14 or 15.30. An optical fibre formed from a preform as claimed in claim 14 or 15,and adapted to exhibit circular birefringence properties.
 31. An opticalfibre formed from a preform as claimed in claim 14 or 15, wherein thepreform is produced from material(s) selected for high opticalnon-linearity, electro- and magneto-optic effect, electricalconductivity and/or optical gain.
 32. A Bragg hole fibre formed from apreform as claimed in claims 14 or 15.